This invention relates to a process for the manufacture of corrosion resistant metal products and to products produced from the process. The invention has particular but not necessarily exclusive application to products comprising a core formed from recycled mild, carbon or stainless steel swarf and having a stainless steel cladding. For example, the invention may also be applicable to a product comprising a core formed from powdered iron ore and even from other metals and metalliferous ores in which the problems identified herein are encountered.
In this specification xe2x80x98swarfxe2x80x99 comprehends the off cuts from machining operations in general and is intended to include the off cuts from turning, boring, shaping and milling operations on engineering steels. The off cuts from a variety of other operations including some stamping and punching operations may also be suitable. For the purposes of this specification, the term includes not only such off cuts composed of raw swarf but also such off cuts from swarf which has been cleaned and/or otherwise treated, for example by the methods described herein, to make them more suitable for forming a billet from which the clad products are made.
The term xe2x80x9cengineering steelxe2x80x9d is intended to describe those low alloy steels which are commonly subjected to machining operations including mild steel (a term which itself includes carbon steel), forging steel and axle or shaft steel all of which contain significant amounts of carbon.
The background of the present invention is set out in detail in the specification of international patent application #PCT/GB94/00091. In that specification reference is made to the specifications of several other patent applications which are discussed further below. One of the products of the process described in the aforementioned application PCT/GB94/00091 which is potentially of commercial and technical importance is a billet comprised of a stainless steel jacket filled with briquettes of mild steel swarf which can be heated and worked into a finished product having the desirable properties and low cost of mild steel but which has a stainless steel cladding for substantially increased corrosion resistance. Attempts to produce such products have not been as successful as was originally expected and it is an object of the present invention to address at least one of the problems which has contributed to this lack of success.
In the numerous experiments which have been conducted in attempts to produce such products, they have persistently exhibited a green chrome oxide layer occurring on the inner face of the stainless steel cladding and at the interface between the cladding and the core. This green layer has occurred despite the fact that metallographic examination of the core after the billet has been heated and rolled indicates substantially complete reduction of all surface oxides in the swarf and substantially complete fusion between the particles of swarf. Bonding between the cladding and the core cannot be relied on where this green layer occurs.
It is thought that chrome oxides on the stainless steel pipe form a barrier between the core of compressed swarf and the stainless steel. This barrier forms during heating and subsequent hot rolling and impedes bonding between the core and the cladding in the final product, To overcome this problem efforts have been directed at reducing or preventing the formation of chrome oxides on the stainless steel pipe. One technique which has been used is aimed at limiting the original oxide/oxygen content within the pipe, before heating commences. Application PCT/GB94/00091 discloses a technique aimed at eliminating surface oxides in the swarf by passing the swarf through a direct-reduction type kiln similar to the kilns used in the production of direct-reduced sponge iron in the production of steel. The equipment and plant required for this process are costly.
In another technique described in application PCT/GB94/00091, the Boudouard equation is suppressed by taking steps aimed at ensuring that reducing gases are present in the billet throughout heating. These steps include the addition of additives to the swarf which generate reducing gases in the billet when it is heated. The additives should not leave behind significant quantities of solid deposits which would later appear as inclusions which would affect the quality of the final product. The additives proposed include urea and ammonium chloride.
To date, the two aforementioned techniques have generally been used together. However, despite the use of these techniques, some degree of oxidation has continued to occur. Although the final product is often generally acceptable for some purposes, the level of rejects due to the unpredictable degree of bonding between the core and the jacket during rolling remains unacceptably high from a commercial point of view. The rejects exhibit excessive spreading of the cladding during the hot rolling process. This severely hinders efficient rolling of the product by limiting the reduction per rolling pass to only light draughts. This limitation causes excessive cooling of the product which in turn reduces bond strength and limits the number of sizes and shapes which can be rolled. Unpredictable bonding between the core and the stainless steel may also be manifested by elongation of the core which, in some cases, can protrude from the centre of the billet. When this happens, further rolling is prevented and the billet must be scrapped. This problem has been addressed by welding short lengths of mild steel pipe (about 100 mm long) to each end of the stainless steel pipe (which generally has been about 200 cm long). The mild steel ends arc crimped closed prior to loading the billet in the furnace. These mild steel ends are thought to act in two ways.
The coefficient of expansion of stainless steel is greater than that of mild steel which causes the pipe to separate from the core due to differential expansion. There is no significant such separation between the mild steel end portions and the core. The mild steel ends thus form with the core a type of xe2x80x9cplugxe2x80x9d at each end of the billet. The compressed mild steel core, furthermore welds very easily to the mild steel pipe ends during initial rolling, thus preventing the escape of the core from the billet during rolling. The use of these mild steel ends is described in detail in international patent application #PCT/GB90/101437. It is not known how effective these plugs are in preventing the ingress of oxidising gases further into the billet as the core is initially still porous. Perhaps only the end portions of the stainless steel pipe are oxidised due to atmospheric oxygen which penetrates the end portions of the billet.
Another advantage of using the mild steel pipe ends is that they facilitate entry of the billet into the rolls, particularly in the first pass.
It is time consuming to cut and weld the mild steel pipe ends to the stainless steel pipes. Furthermore, good quality welds are required to prevent the welds from breaking during hot rolling which would in turn cause oxidation and, at times, scrapping of the billet.
In summary the disadvantages of the techniques described above include:
a costly reduction kiln required to pre-reduce the swarf;
a commercially unacceptable level of rejects due to unpredictable bonding during rolling;
limitations in the sizes and shapes that can be rolled with the billets;
the added cost of welding the mild steel ends onto the stainless steel pipes.
The unpredictability of the described oxidation prevention techniques is thought to be due to the sequence of events which occurs during heating up of the billet.
In the initial phase of heating both NH4Cl and urea generate considerable volumes of reducing gases in the temperature range from 200xc2x0 C. up to about 500xc2x0 C. These gases are expelled from the billets as flames which are visible in the furnace in this temperature range. These flames usually cease abruptly when all of the NH4Cl or urea has evolved into gas and the reaction has gone to completion. Both NH4Cl and urea are spent at well below 600xc2x0 C. Once spent, neither of these substances generates positive pressure inside the billet.
Above 500 and even 600xc2x0 C. there are still reducing gases present from the reaction inside the billet but these are thought to gradually diffuse out of the billet. Furthermore, the volume of such residual reducing gases can be reduced rapidly by a reduction in temperature which brings about a sudden contraction in the volume of gases in the billet. This volume reduction has the effect of sucking in gases which are present in the furnace atmosphere and which are usually if not always oxidising.
The remaining residual reducing gases may be insufficient to neutralise any oxidising gases inside the billet. In the 800-1250xc2x0 C. temperature range the reducing gas is thought to be mostly CO. The billet is especially susceptible to sudden cooling when it is taken out of the furnace in the 10-15 seconds before entering the rolling mill. At this time, significant oxidation can occur from the ends of the billet especially if they are open to the atmosphere.
Three temperature phases have thus been identified and examined during the heating of the billets. The first temperature phase lies in the range from ambient to just over 500xc2x0 C. When NH4Cl or urea is the additive, a reducing gas is generated which scours and purges residual oxygen and some oxides from the system with the object of suppressing the Boudouard equation. This would otherwise create an equilibrium of oxidising gases up to 800xc2x0 C. NH4Cl has been found to be the most effective reducing agent in the first temperature phase even though it reacts for a relatively short part of the total heating cycle, as it disassociates initially into ammonia and hydrochloric acid at below 300xc2x0 C. Hydrochloric acid is a reducing/scouring agent and ammonia disassociates into hydrogen and chlorine at about 500xc2x0 C. Above this temperature the ammonia is completely spent. Several experiments, in which billets were heated only to this temperature, have revealed that the inside walls of the stainless steel pipe were still metallic and not oxidised. Some reduction of the mild steel core had occurred.
The second temperature phase lies in the range 500-800xc2x0 C. It is thought that some of the reducing gases from the first temperature phase are still present during this second phase. It is however believed that the billet is most vulnerable to oxidation in the second phase because conditions inside the billet favour the formation of CO2 (rather than CO) from any iron oxides in the swarf or from any oxidising furnace gases which enter the billet. Carbon occurs in the billet as a result of the decarburisation of the steel of which the core is composed. Even an excess of carbon present in the system will result in an atmosphere which is predominantly CO2. According to the Boudouard equation, such an atmosphere is oxidising to the stainless steel. Mild oxidation of the steel core is not the problem, as such oxidation would be reduced in the following temperature phase.
However, chrome oxide formed in the second temperature phase would not be reduced in the third temperature phase when the temperature ranges from 800-1250xc2x0 C. In this latter phase, in equilibrium according to the Boudouard equation, conditions favour the formation of CO. An atmosphere composed predominantly of CO is highly reducing to carbon steel but at best is thought to be non-oxidising (i.e. neutral) to the stainless steel Numerous experiments have been carried out on billets in which ammonium chloride by itself was used as the additive. In some cases heating has been terminated in the third temperature phase first at 1000xc2x0 C. and then at 1200xc2x0 C. Examination has yielded variable results. The billets have shown mild to marked greenish oxide formation (indicating chrome oxides) on the inner walls of the stainless steel pipe. Such chrome oxides would undoubtedly hinder bonding during subsequent rolling.
The step of providing a reducing agent comprising solid ammonium chloride or urea in the billet is the subject of the invention defined in international patent application #PCT/GB94/00091.
In one aspect of the invention, there is provided a method of producing a corrosion resistant ferrous product in which a billet which comprises a mass of particulate material composed substantially of engineering steel in a stainless steel jacket is heated to a temperature at which the billet can be plastically worked, the method being characterised in that it includes the step of providing in the jacket a first reducing agent in the form of a metal having a greater affinity for oxygen than chrome and a second reducing agent which is present in gaseous or vapour form in the jacket at a temperature substantially below 800xc2x0 C.
In another aspect of the invention, there is provided a method of producing a corrosion resistant ferrous product in which a billet which comprises a mass of particulate material composed substantially of engineering steel in a stainless steel jacket is heated to a temperature at which the billet can be plastically worked, the method being characterised in that it includes the step of providing in the jacket a first reducing agent selected from the group comprising aluminium, titanium, zirconium, magnesium and sodium and a second reducing agent which is present in gaseous or vapour form in the jacket at a temperature substantially below 800xc2x0 C.
Advantageously, the second reducing agent is present at a temperature substantially below 500xc2x0 C.
In one form of the invention the second reducing agent is provided by a substance selected from the group comprising ammonium chloride, urea, iron bromide and ferric chloride. Advantageously, the substance is ammonium chloride.
In an alternative form of the invention, the second reducing agent is derived from a reducing furnace in which the billet is heated.
According to one aspect of the invention, the first reducing agent is in powder form. Advantageously, according to the invention, the first reducing agent is aluminium. Aluminium powder is readily commercially available and inexpensive.
In an alternative form of the invention, the first reducing agent is titanium. Advantageously the titanium is in the form of swarf. According to another aspect of the invention the particulate material is in the form of swarf composed substantially of engineering steel.
Even though aluminium oxidises and results in alumina inclusions in the product, it has been found that the use of this additive results in a higher yield strength steel. No more than 0.06% Al by weight of mild steel swarf is required to add strength.
The first reducing agent and, if it is used, the substance which forms the second reducing agent are advantageously mixed with the swarf before it is compacted in the jacket. The scope of the invention extends to billets produced by the process of the invention and products produced from such billets.
A billet was prepared using mild steel swarf to which was added aluminium powder of 35 mesh size. The amount of powder added was 0.1% of the swarf by weight. The powder was mixed evenly throughout the swarf prior to compression of the swarf in a stainless steel pipe according to the techniques described in PCT/GB94/00091 and the other relevant patent applications discussed therein. The ends of the pipe were closed by welded on end plates. However vent holes were left in the end plates to allow the escape of gases from the interior of the billet when it was heated. The billet was heated to normal rolling temperature of 1250xc2x0 C. in a conventional billet heating furnace. The vent holes were sealed immediately after removal from the furnace. Sealing was effected by welding the vent holes closed. When the billet had cooled, examination of the inside wall surface of the stainless steel pipe revealed some green oxide throughout the inner face of the stainless steel pipe indicating that mild oxidation had occurred.
The conclusion was that aluminium powder added in these conditions and in these quantities was insufficient or in some other way ineffective. Although even at low temperatures, aluminium has a greater affinity for oxygen than chrome, it is likely that the aluminium mixed in the swarf in this way is not sufficiently dispersed to be able to prevent oxidation of the chrome by residual oxygen and by CO2 in the billet derived from decarburisation of the steel and the reduction of iron oxides initially present thereon. Should aluminium be added in greater quantities it is thought that an unacceptably high level of inclusions would result in the finished product.
A billet was prepared using mild steel swarf to which was added a mixture comprising 0.1% by weight of NH4Cl powder and 0. 1% by weight of aluminium powder (again of 35 mesh size). The additives were thoroughly mixed together and evenly distributed throughout the swarf. The billet was then heated as in experiment 1. A characteristic red/yellow flame from the ammonium chloride was observed in the furnace for the initial 30-40% of the time taken for the billet to reach a temperature of 1250xc2x0 C. in the furnace. Inspection of the billet after sealing and cooling as in experiment 1 exhibited an almost completely reduced inner silver stainless steel pipe surface with substantially no trace of green oxides except over a short distance from each end. In these areas the stainless steel was very slightly discoloured, indicating that a small amount of oxidation had taken place on extraction from the furnace and before sealing of the billet ends.
In order to try to avoid the discolouration which occurred in experiment 2, an attempt was made to eliminate the possibility of oxidising gases being sucked into the billet as a result of rapid cooling upon removal from the furnace with consequent reduction of volume of the internal gases. Two billets were prepared by the steps described in experiment 2 except that, three minutes before extracting the billet from the furnace, in a step believed to be inventive, tablets comprising compressed ammonium chloride powder by itself were added to each end of one of the billets before scaling. In the case of a second billet, the tablets comprised a mixture of equal parts of compressed ammonium chloride powder and aluminium powder were added. In both cases, vigorous burning of the tablets was observed on extraction of the billets from the furnace and continued until the vent holes were sealed. The flames emerging from the vent holes were bright white indicating a temperature in the region of 3000xc2x0 C. After the billets had cooled they cut open. Inspection revealed no green oxides on the stainless steel at each end of the cool billets. It appeared therefore that the techniques worked satisfactorily to prevent oxidation of the stainless steel pipe. These techniques combined the effect of oxide reduction in the swarf and the prevention of extraneous oxidising gases from entering the billets. Oxide reduction was achieved by the additives in the swarf. The generation of reducing gases at each end of the billets prevented oxygen (i.e. air) from being sucked into the billets on sudden cooling when the billets were removed from the furnace.
The same experiment, adding pellets comprising aluminium powder alone to the end of a billet, yielded similar results.
Several billets have been prepared for rolling by the techniques set out in experiment 3 and hot rolled into finished products directly after removal from the furnace. In most of billets no mild steel end pieces have been used.
In laboratory conditions, no significant spreading of the cladding in relation to the core nor any significant elongation of the core out of the cladding has been observed. Substantially complete bonding of the cladding and the core in the finished product has been observed.
It is concluded that, by employing the techniques described, no end plugs are required to keep the core in and the oxidising gases out when the billet is removed from the furnace and subsequently hot rolled. Thus the use of mild steel end pieces will not necessarily be essential if the techniques described herein for preventing or reducing the formation of chrome oxides are employed.
Further benefits result from crimping the ends of the billet closed as described in patent application #PCT/GB90/01437. A few minutes prior to removal from the furnace, ammonium chloride and/or aluminium powder, compressed together into large pellets are placed into the two crimped ends. The crimped ends act conveniently as receptacles for the Al/NH4Cl pellets both in solid as well as in melted form. Al/NH4Cl added in this way acts as an oxygen trap or scavenger at the most vulnerable places in the billet which are the open ends.
There is no quantity limitation on the Al/NH4Cl added imposed by concerns for limiting resulting inclusions in the product in this case, because the two ends are always cropped and discarded during the hot rolling process. The aluminium, because it is effective for a longer time than the ammonium chloride, can be added at any stage and in fact could be added regularly throughout the billet heating phases prior to rolling. In fact, aluminium discs may be placed into the two ends of the billet before they are crimped so that the discs act initially to physically restrict the entry of gases into the billet. As the temperature rises, the discs act as reducing agents/oxygen traps and, above 600xc2x0 C., they melt. The molten aluminium is contained in the crimped end portions of the billet which act as receptacles for the aluminium and as efficient oxygen traps as described above. The combined reactions are thought to be as follows:
In the first heating phase (up to 500xc2x0 C.), the predominant reaction is the dissociation of the ammonium chloride when reducing/scouring gases are generated and in part remain present after the reaction is spent.
Even though the aluminium powder is undoubtedly complementing the reducing reaction during this phase, it is thought that it is most effective during the subsequent phases. In the second phase (500-800xc2x0 C.) the aluminium is in its greatest reducing mode. It melts at 600xc2x0 C. thereby suddenly increasing its reactive surface area. In this temperature range, aluminium is an extremely efficient reducing agent as its affinity for oxygen/oxide is greater than that of chrome. Hence, oxidation of the aluminium takes place in preference to oxidation of the chrome in the stainless steel. It is thought that in this phase, when the Boudouard equilibrium would be at its most damaging to chrome, oxidation is either largely suppressed or swings completely towards the carbon monoxide/carbon side of the equation, because any free oxygen/carbon dioxide, and in fact substantially any gases except for the highly reducing gases still present from the first phase are removed from the system by the aluminium.
The next phase (800-1250xc2x0 C.) is probably a continuation of the previous phase, except that the aluminium produces an even stronger reducing reaction with less gaseous phases present. The Boudouard equation strongly favours a carbon monoxide atmosphere above 800xc2x0 C. with the aluminium tending to reduce the carbon monoxide back to carbon in the mild steel. Any oxidising effects on the stainless steel, arising from the Boudouard equation, are largely neutralised. Any carbon monoxide present in the system may act as a reducing gaseous medium with chrome in the presence of aluminium at these temperatures. Oxides present on the steel particles in the core are probably reduced either in the solid phase in proximity with the aluminium powder which is finely dispersed throughout the billet or in the gaseous phase by transient carbon monoxide.
In the final stage the billet is removed from the furnace. In this stage the oxygen scavenging effect of the aluminium combined with the generation of reducing gases from any ammonium chloride added help to ensure that if there is sudden cooling when the billet is removed from the furnace, any gases sucked into the billet are reduced before they are able to oxidise the chrome.
The amount of Al/NH4Cl pellets needed can be visibly determined. If no flames are observed, more pellets could be added prior to removal of the billet from the furnace. Again, no problems arise from the use of too many pellets at the two ends of the billet as these ends are discarded during rolling.
It is thought that graphite would also act to prevent or reduce oxidation in the billets prepared according to the techniques of the invention. Accordingly, powdered graphite may be mixed with the aluminium powder and the ammonium chloride and/or urea if the latter are used. In most cases however, the carbon which diffuses out of the mild steel swarf making up the core when the billet is heated should provide a sufficient source of carbon for this purpose. Engineering steel of up to about 0.45% carbon content should in most cases be suitable for producing products according to the techniques of the present invention.